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Aerodynamic Platform Control

Please enjoy the 2-minute summary gallery below, or scroll further to explore my work in greater detail

Aggressive aerodynamics packages are very sensitive to the attitude and orientation of the vehicle, and perform best when kept inside a tight operating window. These stiff ride requirements often clash with need for a compliant suspension to maintain mechanical grip over the track surface. In this page, I will cover the steps taken to develop the ride platform targets for the 2020 Clemson FSAE vehicle.

The Tiger22 Aero Package

Project Scope

Clemson's first aerodynamics package was implemented in 2017, and the relatively young division still lacks the CFD infrastructure to fully characterize an aerodynamic concept. In the past this has prevented any meaningful platform control analysis, but recent development of an automated full-car CFD script has enabled long-overdue forays into aerodynamic performance sensitivity. A proper characterization would map the car's aerodynamic performance (downforce, drag, and center of pressure) for various speeds, ride heights, pitch, and roll angles, but the only available data for the moment was speed and pitch-angle mapping. 

The available scope of the project is constrained by this key limitation, but there is still meaningful work that can be carried out while the aerodynamics division grows their understanding of the package. This mainly consists of exploring the effect of center of pressure migration under pitch on the performance and stability of the vehicle. With little previous data to refer to, it is difficult to define a definitive "stability target". Instead, the goal is to create a baseline characterization and search for the threshold at which these characteristics diverge to set an upper limit pitch target. This would be combined with existing testing data to generate a complete set of rate and damping targets for the various suspension modes. The final step was to evaluate a set of feasible suspension configurations to select the concept best suited to meet as many of these targets as possible.

Center of Pressure Migration Study

Exploration of Center of Pressure (CoP) migration effects focused on outright braking performance and cornering stability under pitch motions. This was evaluated using a two-track vehicle model with linearized kinematics representation and nonlinear, load-sensitive tires.

Braking performance was a fairly straightforward exploration. Even though total downforce increased with forward pitch angle, this increase was concentrated on the front wing. As the downforce distribution shifted forward, rear tire lock-up became the limiting factor for total braking performance.

This turned out to be a consistently linear effect which became more pronounced at higher velocities. No points of significant divergence were apparent. While the fall-off in brake performance could be addressed with a brake-bias adjustment, we elected to favor low speed brake balance where aero effects are limited.

Although braking performance is incredibly important, maintaining vehicle drivability was a larger priority for me. These metrics were defined using the classical linearized control derivatives for stability, control and response. I evaluated these characteristics across a range of longitudinal acceleration, to represent on track braking and acceleration scenarios. Exploring how these derivatives propagate would provide key insights on how vehicle behavior evolves in a corner entry or corner exit scenario.

First, I studied a vehicle with no aero package in order to create a baseline to compare against:

All of these metrics play an important role in predicting vehicle response to driver inputs. However, my largest priority was to avoid instability under extreme acceleration events. With this in mind, I gave the highest weight to the stability index (bottom right) and yaw returning moment derivative (top middle). With most of the plots, it is evident that mild longitudinal weight transfer has consistent, predictable effects on the balance and stability of the vehicle. On the extreme ends of the x axis, these effects diverge sharply as tires approach their saturation limit, and have very little leftover cornering force available.

Using preliminary CFD results, I created a conservative estimate for CoP migration, which varied roughly linearly with pitch angle, and incorporated that relationship into the model. The plots were generated again, for a full range of predicted track speeds and pitch gradients (units in deg/g).